Winston,
The normal brakes are powered by the right hydraulic system, the alternate brakes from the left. The transfer of braking from the right hydraulic system to the left hydraulic system is automatic with the loss of right system pressure. There is no indication apart from the R HYD SYS PRESS EICAS message and the SYS PRESS light which will give you a clue. However, if the brakes are on the alternate system you can't arm the autobrakes as there is no autobrake with the alternate system.
In normal operation the APU is shutdown in flight. Therefore the aircraft has 2 electrical supplies ie. the engine driven generators. In the case of a double engine failure you will lose electrical power to the electric hydraulic pumps. The Ram Air Turbine will deploy and supply hydraulic power to the centre system and therefore flying controls until either you restart one or both engines or the APU comes on line. Incidentally, it provides adequate pressure to about 130kts.
Hope this helps.
Last edited by EPRman; 11th October 2002 at 21:54.